People have mentioned that the video was taken from a spot just off of the bridge, with Abby being close to but between Libby and BG. I agree with this, and the idea that Libby's phone was down by her right side. thus the camera on the phone was pointed slightly upwards at this short just over one second length of video. GH did a good analysis earlier this year after the video was released. He added the phone movements to the cropped video released by LE.
So we have a cropped section of video to work with, zoomed in on BG, and the phone was moving as the video was being taken. If we had the uncropped, full screen version, what I'm describing would be easily seen.